Microscopic investigation of Bi2-xSbxTe3-ySey systems: On the origin of a robust intrinsic topological insulator. (May 2019)

• Record Type: Journal Article
• Authors: Nam, Hyoungdo; Xu, Yang; Miotkowski, Ireneusz; Tian, Jifa; Chen, Yong P.; Liu, Chang; Hasan, M. Zahid; Zhu, Wenguang; Fiete, Gregory A.; Shih, Chih-Kang
• Is Part Of: Journal of physics and chemistry of solids. Volume 128(2019)
• Page Start: 251
• Page End: 257
• Publication Date: 2019-05
• Subjects: Topological insulator -- BiSbTeSe2 -- Be2Te2Se -- Aging effect -- Dirac point; Solids -- Periodicals; Solides -- Périodiques; Solids; Periodicals; 530.41
• DOI: 10.1016/j.jpcs.2017.10.026
• Languages: English
• ISSNs: 0022-3697

Abstract

Abstract: One of the most important challenges in the field of topological insulators (TI) is to find materials with nontrivial topological surface state (TSS) while keeping the bulk intrinsic (insulating). In this letter, we report microscopic investigations of BiSbTeSe2 (1112) and Bi2 Te2 Se (221) alloys which have been proposed as candidates to achieve an intrinsic bulk. Scanning tunneling microscopy (STM) confirms previous macroscopic experiments that 221 is an ordered alloy with a Te-Bi-Se-Bi-Te sequence. Nevertheless, it also reveals that the ordering is not perfect with the surface chalcogen layer containing 85% Te and 15% Se. On the other hand, STM shows that 1112 is a random alloy with a fine mixture of (Bi, Sb) in the pnictogen layers and (Te, Se) in the top/bottom chalcogen layers. A freshly cleaved 221 sample surface shows an intrinsic bulk with the Fermi energy, EF, in the gap, but quickly becomes n-type with aging, similar to the aging effect reported by others [27, 32]. By contrast, the random alloy 1112 show remarkable robustness against aging and the EF remains within the gap even after aging for 7 days. We attribute this result to the nanoscale fine mixture of the random alloy which provides an effective doping compensation in very fine length scale, thus enabling its bulk to remain intrinsic against aging. Highlights: BiSbTeSe2 has a random structure; whereas, Bi2 Te2 Se has an ordered structure. Despite of the random composition of BiSbTeSe2, Fermi level is located around the Dirac point with the fluctuation of just 34 meV (full width at half maximum). Femi level of BiSbTeSe2 is stably pinned near the Dirac point against aging and defects. Bi2 Te2 Se with an ordered structure showed a dramatic change of Fermi level due to aging effect.